Every model has individual pricing. A basic question is 1 sat to 10 sats. Like all AI usage the larger the context the larger the token usage. Top models need a minimum balance of a few thousand sats to get started. You can deposit cashu or pay with lightning to the mint. Then you can send your remaining balance out after you are done. You don't have to keep any funds with the mint beyond when you use them unless you want. You can create a new nostr npub/nsec every time or create one just for routstr or use your real one if you like.